This invention relates to an improvement in the process for the polymerization of chloroprene to mercaptanmodified polymers.
Polymerization of chloroprene in an alkaline aqueous emulsion in the presence of alkyl mercaptans as chain transfer agents is well known. It has been customary in prior practice to stop chloroprene polymerization at a point at which no significant amount of gel was formed because a significant amount of such gel is a chloroprene polymer adversely affects such processing characteristics as polymer smoothness, die definition, and surface appearance of extruded objects. Furthermore, in the preparation of solvent based adhesives, such as rubber cements, it is undesirable to have present an excessive amount of solvent-insoluble material.
The point at which gel formation becomes apparent in a chloroprene polymerization varies with the temperature of polymerization. For example, at 40.degree.C. in a normal emulsion system, the maximum monomer conversion possible has been in the neighborhood of 65 to 70%. While gel formation can be delayed by lowering the temperature of polymerization, changes in the crystalline nature of the resulting polymer make the polymer less suitable for certain low temperature applications of the final vulcanizate. At 0.degree. to 10.degree.C., polymerization can be carried to a conversion as high as about 90%. These polymers crystallize rapidly and are, therefore, well suited for use in adhesives. Even in these high-conversion polymerizations, it is economically desirable to increase the monomer conversion without producing substantial amounts of polymer insoluble in usual solvents.
There is, therefore, a great need for an economically attractive process for polymerizing chloroprene monomer to a high degree of conversion without sacrificing the desirable properties of the resulting polymer.